1. Field of the Invention
The present invention relates to the field of optics in general. More particularly, the present invention relates to the field of depolarizers which have applications in communications, sensors, optical instruments and other areas.
2. Description of the Related Art
Many optical devices used in communications and instrumentation, such as switches, couplers and modulators, are highly sensitive to the state of polarization of light. The performance of communication systems and instruments which utilize such optical devices varies as the state of polarization (hereinafter SOP) varies. Fluctuations in the SOP can result in reduced signal to noise ratios in fiber optic communication systems or decreased sensitivity and accuracy in fiber optic instruments.
When light passes through a fiber optic cable (hereinafter fiber), the initial polarization of the light, whether polarized elliptically or linearly, can be changed due to varying environmental factors affecting the fiber. These environmental factors produce changes in the index of refraction of the fiber. Light propagating along the fiber will pass through these regions having differing indexes of refraction, thereby changing the initial SOP of the light as it propagates along the fiber. This effect of altering the SOP of light as it passes through a medium is called birefringence. The polarization received at the output end of the fiber may thus change radically from the initial SOP at the input end of the fiber. Because birefringence is affected by varying environmental factors, the output SOP will not have a predictable relation to the input SOP. Instruments and devices sensitive to SOP will therefore have their performance degraded in a manner which cannot easily be predicted or corrected.
One solution to the problem of birefringence is to replace common single mode fiber with polarization maintaining fiber (hereinafter PMF), which is not sensitive to environmental factors and therefore preserves the initial SOP as light propagates along the fiber. While PMF has advantages over standard single mode fiber, it is also very expensive to use. One meter of PMF costs approximately $10.00, roughly 100 times the cost of single mode fiber.
While single mode fiber has the drawback of birefringence, if the incident light propagating within the fiber is depolarized, then the birefringent effect will not alter the SOP. Depolarized light is the combination of light of all polarization states in equal proportion. Birefringence in single mode fiber alters all of the polarization states equally, thereby preserving the depolarization of light propagating along the fiber. Thus, if depolarized light is used, birefringence no longer produces a degradation in system performance.
One problem associated with utilizing depolarized light is that light sources used in fiber optic systems have a high degree of polarization (hereinafter DOP). The DOP is defined as the fraction of optical power that is polarized. To utilize polarized light sources, a depolarizer must be employed to remove the DOP. Currently available depolarizers have significant limitations which reduce their practical applicability in both fiber optic communication systems and fiber optic instrumentation.
One type of depolarizer is the electro-optic pseudo-depolarizer, which utilizes electrodes positioned on either side of a waveguide to change the refractive index within the waveguide. The varying refractive index in turn varies the SOP of the light passing through the waveguide. Although varying the refractive index of the waveguide varies the SOP, the measured effective DOP depends on detector speed. Over several cycles of varying the refractive index of the waveguide, the time-averaged output light appears depolarized in that no one SOP is preferred during the averaging time. This form of depolarization is called pseudo-depolarization or time-averaged depolarization, and has the disadvantage that light exiting the depolarizer within a narrow time interval has a high DOP. High speed detectors, however, detect light in a narrow time interval. Thus, a high speed detector would capture light with a high DOP when the light is time averaged over the narrow time interval. Additionally, the electro-optic pseudo-depolarizer is an active system requiring both driving circuitry and a power supply. Failure of any of these active components would result in the light exiting the waveguide with a high DOP. Another drawback of the electro-optic pseudo-depolarizer is its high cost. An electro-optic pseudo-depolarizer costs approximately $1000.00.
Another type of currently available depolarizer is the acoustic depolarizer. A driving speaker vibrates a segment of fiber within the depolarizer, thereby altering the index of refraction within the fiber as the fiber bends and vibrates. The index of refraction within the fiber varies at the frequency of the speaker. Polarized light passing through the vibrating fiber has its SOP altered at the frequency of the speaker. As with the electro-optic pseudo-depolarizer, the acoustic depolarizer depolarizes light on a time averaged basis. The DOP then varies at the frequency of the driving speaker. For detectors and instruments which detect polarization states at a time interval narrower than the time interval for depolarization, which is dependent on the frequency of the speaker, light exiting the acoustic depolarizer will have a noticeable DOP. Another disadvantage of the driving speaker depolarizer is that it is an active system which relies on the performance of the driving speaker. In addition to the costs associated with the driving speaker and associated circuitry, such a system is prone to failure if any of the many components of the speaker or driving circuitry fail. Thus, although the driving speaker depolarizer reduces the DOP, the output light still retains a significant DOP within a narrow time interval.
Another known depolarizer is the Lyot depolarizer. The Lyot depolarizer consists of two plates of quartz crystal having large retardances. The light source utilized with a Lyot depolarizer is a broad band source, for example a superluminescent diode. The crystals are arranged such that the incident light passes through the first crystal and into a second crystal adjacent to the first crystal. The ratio of thickness of the two crystals is 2:1. While the light exiting the second crystal is depolarized over a large wavelength region, light in a small wavelength region is not depolarized. Thus, the Lyot depolarizer is ineffective for depolarizing monochromatic or narrow wavelength light sources. Another drawback of the Lyot depolarizer is the high cost associated with using a broad band light source. Broad band light sources have the additional disadvantage of having lower output power than is possible with narrow band light sources. Another disadvantage of the Lyot depolarizer is its inapplicability with many fiber optic communication systems due to the use of a broad band light source. As a pulse of light from a broad band source propagates along the birefringent single mode fiber used in many communications systems, there is a time shift in the pulse caused by the different propagation speeds of the different wavelength components of the broad band light pulse. This time shift causes a "spreading" of the light pulse and is incompatible with the high data transmission rates of many fiber optic communication systems.
Still another type of currently available depolarizer is described in U.S. Pat. No. 5,486,916, issued to Michal et al., and in U.S. Pat. No. 5,457,756, issued to Hartl et al. This type of depolarizer is constructed from PMF. The ends of the PMF are oriented such that their principal axes are at an angle of 45.degree.. In such a depolarizer the quality of the depolarizer depends critically on the 45.degree. alignment of the PMF. As with the Lyot depolarizer, this type of depolarizer also requires a broad band light source. Thus, in addition to the high cost of the depolarizer dictated by the use of a broad band light source and the use of PMF, there are high fabrication costs associated with critically aligning and fusing the PMF. As with the time averaging pseudo depolarizers described above, this depolarizer spectrum averages and has the disadvantage in that it cannot be connected in series with other depolarizers of the same type. If it is connected in series with itself, as when two depolarizers are arranged such that the output of one depolarizer is input a second depolarizer, the output light from the second depolarizer has its DOP increased from the output of the first depolarizer. Thus, the depolarizer described in U.S. Pat. No. 5,486,916 is not suitable for producing very low DOP through series combination of the depolarizer.
Accordingly, it is desired that the present invention overcome the limitations of current optical depolarizers.